The Unity of Consciousness

Human consciousness usually displays a striking unity. When one
experiences a noise and, say, a pain, one is not conscious of the
noise and then, separately, of the pain. One is conscious of the noise
and pain together, as aspects of a single conscious experience. Since
at least the time of Immanuel Kant (1781/7), this phenomenon has been
called the unity of consciousness. More generally, it is
consciousness not of A and, separately, of B and,
separately, of C, but of
A-and-B-and-C together, as the contents of
a single conscious state.

Historically, the notion of the unity of consciousness has played a
very large role in thought about the mind. Indeed, as we will see, it
figured centrally in some of the most influential arguments about the
mind from the time of Descartes to the 20th century. In the
early part of the 20th century, the notion largely
disappeared for a time, but since the 1960s, analytic philosophers and
others have begun to pay attention to it again.

1. History

The unity of consciousness was a main concern of most philosophers in
what is often called the ‘classical modern era’ (roughly,
1600 to 1900), including Descartes, Leibniz, Kant, Hume (in a way; see
below), Reid, Brentano, and James.

Consider a classical argument of Descartes’ for mind-body
dualism. It starts like this:

When I consider the mind, that is to say, myself inasmuch as I am only
a thinking thing, I cannot distinguish in myself any parts, but
apprehend myself to be clearly one and entire. [Descartes 1641: 196]

Descartes then asserts that if the mind is not made of parts, it
cannot be made of matter because anything material has parts. He adds
that this by itself would be enough to prove dualism, had he not
already proven it elsewhere. Notice where it is that I cannot
distinguish any parts. It is in “myself inasmuch as I am only a
thinking thing” (ibid.); that is, in myself as a
whole—which requires unified consciousness of myself as a whole.
The claim is that this subject, the target of this unified
consciousness, is not a composite of parts.

In Kant (1781/7), the notion that consciousness is unified is central
to his ‘transcendental deduction of the categories’ (see
the entry on
Kant’s view of the mind and consciousness of self
for a fuller treatment of Kant). There Kant claims that in order to
tie various objects of experience together into a single unified
conscious experience of the world, we must be able to apply certain
concepts to the items in question. In particular, we have to apply
concepts from each of four fundamental categories of concept:
quantitative, qualitative, relational, and what he called
‘modal’ concepts. Kant’s attempt to link the unity
of consciousness to the structure of knowledge continues to capture
the imaginations of philosophers: Arguments of this form can be found
in P. F. Strawson (1966), Cassam (1996), Hurley (1994, 1998) and
Revonsuo (2003), and are examined critically in
Section 7.3
and in Brook (2005).

Kant was familiar with arguments of the kind that we just saw
Descartes mount (chiefly from similar reasoning in Leibniz and
Mendelssohn) but he was not impressed. For Kant, that consciousness is
unified tells us nothing about what sorts of entity minds are,
including whether or not they are made out of matter (1781, chapter on
the Paralogisms of Pure Reason). He argues that the achievement of
unified consciousness by a system of components acting together would
be no more or less mysterious than its being achieved by something
that is simple, i.e., has no components (1781: A352).

Leibniz, Hume, Reid, Brentano, and James held a variety of positions
on unity. Briefly, for Leibniz (see the entry on
Leibniz’s philosophy of mind)
unified consciousness and the noncompositeness, the indivisibility
that he took to be required for it seem to have served as his model of
a monad, the building block of all reality. With Hume (1739), things
are more complicated. It should have followed from his atomism that
there is no unified consciousness, just “a bundle of different
perceptions” ([1739] 1962: 252). Yet, in a famous appendix, he
says that there is something he cannot render consistent with his
atomism (p. 636). He never tells us what it is but it may have been
that consciousness strongly appears to be more than a bundle of
independent ‘perceptions’. Reid (1785), almost an exact
contemporary of Kant’s, made extensive use of the unity of
consciousness, among other things to run Descartes’ argument
from unity to indivisibility the other way around. Brentano (1874)
argued that all the conscious states of a person at a time will and
perhaps must be unified with one another. (He combined this view with
another strong thesis, that all mental states are conscious.) Finally,
late in the 19th century James developed a detailed
treatment of synchronic (or ‘at a time’) unity of
consciousness. We will discuss his view later (see also entries on
David Hume,
Thomas Reid,
Franz Brentano,
and
William James).

Early in the 20th century, the unity of consciousness
almost disappeared from the research agenda. Logical atomism in
philosophy and behaviourism in psychology had little to say about the
notion. Logical atomism focussed on the atomic elements of cognition
(sense data, simple propositional judgments, protocol sentences,
etc.), rather than on how these elements are tied together to form a
mind. Behaviourism urged that we focus on behaviour, the mind being
either a myth or at least something that we cannot and do not need to
study in a science of the human person.

One partial exception to this pattern of neglect was Gestalt
psychology. Indeed, Gestalt psychology was sufficiently influential in
its time that some positivists tried to make their systems compatible
with it (Smith 1994: 23). For instance, Carnap chose to avoid any
commitment to atoms of experience as the elements of his system,
opting instead for ‘total experiences’. As we will see, a
notion similar to his concept of irreducible experiential wholes can
be fruitful
(Section 7.4).
However, Carnap seems to have had something rather different in mind
from what philosophers now have in mind when they speak of the unity
of consciousness. Gestalt unity is a unity in a structure of which one
is conscious, where the way in which each part appears is derived from
the structure of the whole (Tye 2003: 11–5; Bayne & Chalmers
2003: 27). This is distinct from unity in one’s
consciousness of objects, objects that need not themselves
exhibit the qualities of gestalt structures.

After decades of neglect, the last third of the 20th
century saw a resurgence of interest in unified consciousness among
analytic philosophers. It began with influential commentaries on Kant
in the 1960s (Strawson 1966; Bennett 1966, see also his 1974), as well
as discussions by Nagel (1971) and Parfit (1971, 1984). More recently,
quite a number of philosophers and a few psychologists have written on
the subject, including Marks (1981), Trevarthen (1984), Lockwood
(1989, 1994), Hill (1991), Brook (1994), Marcel (1994), Hurley (1994,
1998), Shoemaker (1996, 2003), O’Brien and Opie (1998). The first
decades of the 21st century have seen a lot of important
new work on the subject: Dainton (2000), Stevenson (2000), Bayne and
Chalmers (2003), Hurley (2003), Kennett and Matthews (2003), Rosenthal
(2003), Radden (2003),
Tye (2003), Zeki (2003), Nikolinakos (2004), Brook and Raymont (2006),
LaRock (2007), and Bayne (2008, 2010, the latter a book-length study).
Cleeremans (2003) is an excellent collection containing papers by
philosophers such as Bayne, Chalmers, Hurley, Shoemaker, Cotterill,
and Thompson and psychologists such as Triesman, Humphreys, Engel,
Diennes, Perner, and Varela. Blackmore (2004, especially ch. 17) is a
good, scientifically-oriented introduction. The section on the unity
of consciousness in PhilPapers has 391 entries (as of April 2017), the
vast majority from the last twenty years (see
Other Internet Resources).

2. Characterizations and Taxonomies

2.1 Characterizations

What characterizes the unity of consciousness? Note that this question
can be asked and answered whether or not there is any such
thing as unified consciousness. Indeed, we need to know what the unity
of consciousness is like even to address the question of its
existence. (For ease of exposition, we will write as though there is
unified consciousness, even though the question really remains open
until the next section.)

That said, it should also be noted that it is difficult to say much
about the unity of consciousness that is both non-question-begging and
more than a thinly disguised synonym, a point that Dainton (2000)
emphasizes. Even as great a theorist of the subject as Immanuel Kant
threw up his hands. He observed that this unity is “not the
category of unity” (B131), that is to say, is not just a matter
of being numerically one—and said no more.

Underlying the various attempts to identify what is characteristic
about the unity of consciousness are two opposing views of the
structure of a unified conscious experience. On what we will call the
experiential parts view (EP), a unified conscious experience is a
composite of other experiences. The no experiential parts view (NEP)
denies this, asserting that while a unified conscious experience will
have a complex object or content, it has no experiential parts. On
this view, when the objects of particular experiences get incorporated
into a ‘bigger’ unified experience, the new experience
replaces the particular experiences rather than containing them as
parts. It would be premature to discuss the two views here (see
Sections
7.1
and
7.2)
but we need to know about them to understand the attempts that have
been made to characterize unity. The first two ways of characterizing
the unity of consciousness that we will examine are within the
experiential parts approach.

2.1.1 Subsumption

One increasingly prominent attempt to characterize the unity of
consciousness holds that in unified consciousness, particular
experiences are subsumed in a more complex experience. For example,
Bayne and Chalmers (2003) say that when particular experiences are
unified, they are “aspects of a single encompassing state of
consciousness” (see also Bayne 2010: 20, 31). More precisely,
two experiences are what they call ‘subsumptively unified’
“when they are both subsumed by a single state of
consciousness” (2010: 27). This yields a distinctive
phenomenology. Two subsumptively unified states will have what they
call a conjoint phenomenology: a phenomenology of having both states
at once that subsumes the phenomenology of the individual states:
“there is something it is like for the subject to be in [two
conscious] states simultaneously” (2010: 32).

One feature of subsumption is that it requires there to be
experiential parts. Thus, those who favour NEP or even wish a
characterization of unified consciousness to be neutral on this issue
will look for a different account.

2.1.2 Co-Consciousness

A second attempt to characterize unified consciousness claims that a
relation among local conscious states is the crucial element, a
relation usually called co-consciousness. As James
put it, in synchronic unified consciousness, we are co-conscious of
A, B, and C (1909: 221). Others who centre their
analysis on the notion include Parfit (1984) and Hurley (1998). These
theorists seldom try to define the term
‘co-consciousness’. They treat the notion as being
intuitively clear and let it function as a primitive in their
analysis.

Like subsumption, most versions of co-consciousness require
experiential parts (James, who accepted NEP and thus had an unusual
conception of co-consciousness, is an exception). In addition to a
problem of lack of neutrality, this requirement faces the problem that
some forms of unified consciousness do not seem to involve
multiplicity of items, unified consciousness of self for example. If
there is no multiplicity of items, there is nothing to enter into a
‘co’-relationship.

A number of theorists combine the two approaches. Many of the people
who use the term ‘co-consciousness’ in fact seem to have
subsumption in mind as the underlying notion. Dainton, for example,
embraces the language of co-consciousness and relates the notion
to“being experienced together” [subsumption] (2000: 236)
but urges in the end that co-consciousness is a better primitive than
subsumption (Dainton 2005: 258–259). Or Lockwood (1989: 88):
co-consciousness is “the relation in which two experiences
stand, when there is an experience of which they are both
parts”. Similarly Shoemaker (2003: 65): “The experiences
are co-conscious … by virtue of the fact that they are
components of a single state of consciousness … ”.

2.1.3 Joint Consciousness

There are at least two approaches to what characterizes unified
consciousness that are compatible with NEP. One we find in Tye. In
unified conscious states, the things that we experience are
“experienced together”, “enter into the same
phenomenal content” (2003: 36, his
emphasis)—which phenomenal content could be the content of a
single non-composite experience.

Another has been advanced by Brook and Raymont (Brook 1994: 38; Brook
2000; Brook & Raymont 2006). The key idea is what they call joint
consciousness. Joint consciousness is present when the following
holds: If an experience that one is having provides consciousness of
any item, then it provides consciousness of other items and of at
least some of the items as a group. Likewise for consciousness of acts
of experiencing.

This notion tries to capture what is distinctive about unified
consciousness in a way that is neutral with respect to the EP/NEP
debate. The notion is related to the phenomenal side of the
Bayne/Chalmers notion of subsumption, there being something it is like
to be in two conscious states simultaneously (Bayne & Chalmers
2003: 32), but in a way that is free of the non-neutral notion of
subsumption. It is not clear whether joint consciousness is an
alternative to Tye’s notion of same phenomenal content or an
attempt to say something about what yields such content.

2.2 Taxonomies

Most contemporary theorists agree that unified consciousness can take
a number of forms. Many schemes for dividing it up exist in the
literature. Tye (2003: 11–5), for example, distinguishes object
unity, neurophysiological unity, spatial unity, subject unity,
introspective unity, and, finally, phenomenal unity. The latter is the
notion that he explicates in terms of contents being experienced
together, entering into the same phenomenal content, and is the notion
on which he focuses.

Similarly, Bayne and Chalmers (2003: 24–7) distinguish objectual
unity (a matter of two conscious experiences of one object, so
different from Tye’s object unity), spatial unity, subject
unity, and subsumptive unity, the last a matter of two or more
conscious states becoming aspects of a single conscious state. Then
within subsumptive unity, they distinguish between access unity and
phenomenal unity. We just examined their definition of the latter. As
is true of Tye, it is what mainly interests them. Two conscious states
are phenomenally unified “if there is something it is like for
the subject to be in both conscious states simultaneously”.

Kant and philosophers in the Kantian tradition break phenomenal unity
down. The division usually follows the traditional division of
experience into subject, representation, and object or content,
assigning to each its own form of unified consciousness. Thus there
will be unified consciousness of individual objects, of multiples of
objects, of acts of experiencing, and of oneself as the subject of
such experiencing. (A fifth form can be distinguished, too, as we will
see.) Few contemporary theorists break phenomenal unity down at all,
so this division is of some interest.

The first three forms of unified consciousness in the Kantian
tradition can be expressed in terms of the notion of joint
consciousness just introduced. First, unified consciousness of
individual objects. This is Tye’s object unity; Bayne and
Chalmers’ objectual unity is at least a related notion. The
process at work here is now commonly called binding (Hardcastle 1998;
Revonsuo 1999). Binding is the process of tying various features of a
visual scene such as colour, shape, edges, and contours, features
detected in various places in the visual cortex, together into an
experience of a unified, three-dimensional object. Binding may be
necessary for consciousness of individual objects but it does not seem
to be sufficient. We must, it seems, also be jointly conscious of the
various elements to have unified consciousness of an object.

Next, unified consciousness of contents. In unified
consciousness of contents, if an experience that one is having
provides consciousness of any object or content, then it provides
consciousness of other objects or contents and of at least some of the
items as a group. Here ‘object’ and ‘item’
cover objects, properties, events—anything of which one can be
conscious. (We speak of experiences rather than representations in
deference to those who doubt that we experience in representations, or
need do so: we wish to be neutral on this issue.)

This distinction between unified consciousness of individual objects
and of multiples of objects corresponds closely to two kinds of
synthesis distinguished by Kant (1781/7, First Division, Book 1,
Chapter 2). Kant distinguishes between the kind of acts of synthesis
needed to attain consciousness of individual objects and the kind of
acts of synthesis needed to attain consciousness of a number of
objects at the same time as a single array of objects experienced by a
single subject (Brook 1994: 123). He builds his argument for necessary
causal connectedness on the latter.

Unified consciousness of contents appears to be central to our kind of
consciousness. For example, suppose that one is conscious of
one’s computer and also of the car sitting in one’s
driveway. If consciousness of these two items were not unified, an
important, indeed probably the most important, way of comparing them
would not be available. One could not answer questions such as, Is the
car the same colour as the WordPerfect icon?, or even, Is the car to
the left or to the right of the computer? That is what unified
consciousness does for us: it allows us to make such comparisons.
Since relating item to item in this and related ways is fundamental to
our kind of cognition, unified consciousness is fundamental to our
kind of cognition. As we will see in
Section 4.1,
there are disorders of consciousness in which this ability to compare
seems to be lost. These disorders leave people with a massive
cognitive impairment.

Most theorists outside of philosophy and many within accept that there
is a second form of conscious unity related to unified consciousness
of contents, namely, unified consciousness of acts of
experiencing. It is present when, for the current acts of
experiencing that one is doing, consciousness of one act of
experiencing (consciousness of how one is experiencing something, for
example seeing it, imagining it, …) provides consciousness of other
acts of experiencing. (This explication is structured to be neutral as
to whether unified conscious states include a multiplicity of
conscious states. We speak of ‘consciousness of acts of
experiencing’ rather than ‘consciousness of an
experience’ for the same reason.)

Not all theorists accept that this second form of unified
consciousness exists. Those who promote the so-called transparency
thesis, the claim that we are not directly conscious of our own
experiencings, deny that we have any such form of consciousness
(Dretske 1995; Tye 2003). Tye, for example, says that when we hear
something, we are not conscious of the auditory experience, just what
it represents. If one tries to be conscious of the experience, at best
one is aware only of “the auditory qualities that the experience
represents” (2003: 33).

Many theorists have also had a fourth thing in mind when they speak of
the unity of consciousness, namely, unified consciousness of
oneself, the thing that has the experiences. Here, one is or
certainly seems to be (see the discussion of Rosenthal in
Section 3.1)
conscious of oneself not just as subject but, in Kant’s words
(A350), as the ‘single common subject’ of many or all the
aspects of the unified experience that one is now having and of a
number of similar experiences past and, in anticipation, still to
come. (Mutatis mutandis, the same holds for the single common
agent of various bits of deliberation and action.) One has unified
consciousness of self when one is conscious of oneself as the single
common subject of experiences of many items in many acts of
experiencing.

The unified consciousness here seems not to be a matter of joint
consciousness. When one is conscious of oneself as the common subject
of one’s current unified acts of experiencing and of unified
acts of experiencing past and to come, one is not conscious of a
number of objects, nor a number of acts of experiencing either.
(Indeed, if Kant is right, when one is conscious of oneself as
subject, one need not be conscious of oneself as an object at
all (A382, A402, B429).) However, something similar might be at work.
One seems to be conscious of one and the same thing as one and the
same thing, namely, oneself, via a number of acts of
experiencing.

Unified consciousness of self has been argued to have some very
special properties, for example that the reference to oneself as
oneself by which one achieves consciousness of oneself as subject must
be indexical and cannot make use of ‘identification’
(Castañeda 1966; Shoemaker 1968; Perry 1979). Generalizing the
latter notion, it has been claimed that reference to self does not
proceed by way of attribution of properties or features to oneself at
all (Brook 2001). One argument for this view is that one is or could
be conscious of oneself in the same way as the subject of each and
every one of one’s conscious experiences. If so, one would not
be conscious of oneself as one kind of thing rather than another. As
Bennett (1974: 80) once put it, consciousness of self would not be
‘experience-dividing’—statements expressing it would
have “no direct implications of the form ‘I shall
experience C rather than D’”. And if that is
so, one’s consciousness of self might not be gained via being
conscious of features of oneself at all

Some theorists especially in the empirical literature hold that a
fifth form of phenomenal experience should attract the label,
‘unified consciousness’. We might call it unity of
focal attention. It differs from the other forms of unified
consciousness that we have delineated. In the others, consciousness
ranges over either many experienced items (unified consciousness of
contents), experiencings of many objects (unified consciousness of
experiencing), or multiple acts of access to oneself as subject of
many experiencings (unified consciousness of self). Unity of focus
picks out something within these unified ‘fields’. Wilhelm
Wundt captured what we have in mind in his distinction between the
field of consciousness (Blickfeld) and the focus of
consciousness (Blickpunkt; Wundt [1874] 1893: Vol. II, 67).
The consciousness of a single item on which one is focussing is
unified because one is conscious of many aspects of the item
in one state or act of consciousness (especially relational aspects,
e.g., any dangers it poses, how it relates to one’s goals,
etc.), and of many different considerations with respect to that item
(one’s goals, how well one is achieving them with respect to
this object, etc.), in the same state or act of consciousness. In
unified focal attention, one integrates a number of cognitive
abilities and applies them to an object. Bayne and
Chalmers’ objectual unity is a related notion (2003:
24–25). Note that, if there are forms of unified consciousness
different from focal attention, then, contrary to Posner (1994) and
others, attention is not a component of all forms of consciousness
(Hardcastle 1997).

All five of the forms of phenomenal unity can, to one degree or
another, be attributed to Kant. The first four are clearly Kantian but
there is a connection to him even with respect to the fifth. He
didn’t speak of attention very often but he did speak of it.
See, for example, B156n.

Since Hume ([1739] 1962: 252) and Rosenthal (2003) deny that we have
such unified consciousness of self and Dennett (1991, 1992) says at
minimum that we have much less of it than we think, it is perhaps
pertinent to say again that in this section we are merely trying to
say what the various forms of unity would be like if they exist.
Whether they do exist is the topic of
Section 3.

2.3 Other Forms of Mental Unity

We will close this section by noting that the forms of unified
consciousness distinguished above are not the only kinds of mental
unity. Earlier we mentioned Gestalt unity. There is also
unity in the exercise of our cognitive capacities, unity that consists
of integration of motivating factors, perceptions, beliefs, etc., and
there is unity in the outputs, unity that consists of integration of
behaviour.

Human beings bring a strikingly wide range of factors to bear on a
cognitive task such as seeking to characterize something or trying to
reach a decision about what to do about something. For example, we can
bring to bear: what we want; what we believe; our attitudes to self,
situation, and context; input from each of our various senses;
information about the situation, other people, others’ beliefs,
desires, attitudes, etc.; the resources of however many languages we
have available to us; various kinds of memory; bodily sensations;
diverse problem-solving skills; and so on. Not only can we bring all
these elements to bear, we can integrate them in a way that is highly
structured and ingeniously appropriate to our goals and the
situation(s) before us. This form of mental unity could appropriately
be called unity of cognition.

It is plausible to hold that unity of cognition is required for unity
of focal attention. However, there is at least some measure of unified
cognition in many situations of which we are not conscious, as is
attested by our ability to balance, control our posture, maneuver
around obstacles while our consciousness is entirely absorbed with
something else, and so on.

At the other end of the cognitive process, we find an equally
interesting form of unity, what we might call unity of
behaviour: our ability to coordinate our limbs, eyes, bodily
attitude, etc. The precision and complexity of the behavioural
coordination we can achieve would be difficult to exaggerate. Think of
a concert pianist performing a complicated work. However, this
capacity to unify behaviour, though doubtless a product of unified
consciousness, does not figure in what unified consciousness is.

3. Is Consciousness Unified?

Now that we know what we are talking about when we talk about unified
consciousness, the next question to ask is: Does it exist?
Does consciousness have the properties that it would need to
have to be unified? If this division of questions looks peculiar,
notice that it can apply to Santa Claus, too. We can develop an
account of what Santa Claus would be like without committing ourselves
on the question of whether such a being exists. However, the division
may look peculiar in another way: How could anyone deny that
consciousness is unified? That it is seems just obvious. In fact,
there has been a good deal of skepticism on the matter.

Some will urge that before we ask whether unified consciousness
exists, we should first ask, Does consciousness exist? There
now seems to be a wide consensus that the answer to this question is,
Yes, there is in us something appropriately called
‘consciousness’. Even those who hold that the
long-standing idea that
intentionality (see entry)
is a matter of attitudes to propositions is false and ripe for
elimination, Paul and Patricia Churchland for example, allow that
consciousness exists, though they urge that the concept be trimmed a
bit (see, for example, Patricia Churchland 1983). Some writers have
taken Dennett (1991) to deny that consciousness exists, either
directly or by implication. He himself has said repeatedly that
consciousness is real, however (1995: 135, 146 are two
examples). A few writers, Wilkes (1984) and Rey (1988) for example,
have espoused true eliminativist about consciousness but they are a
tiny minority.

3.1 Skepticism about Unity

Many philosophers have been sceptical about whether consciousness is
unified. It is possible to be sceptical about whether all
consciousness is unified, whether as many conscious states are unified
as we might think, and, the strongest form of scepticism here, whether
there is any unity of consciousness. Let us examine the three in
reverse order.

Hume ([1739] 1962: 252) seems to have been an example of the strongest
form of scepticism. He famously doubted, or tried to doubt, that we
have unified consciousness even of the self, let alone of one’s
conscious states. Among recent writers, perhaps the most sceptical
about consciousness being unified is Rosenthal. Rosenthal holds that
all we have is a “sense of the unity of
consciousness” (1986: 344, emphasis added). Why merely a sense?
On his view,

Mental states are conscious, when they are, in virtue of their being
accompanied by HOTs [higher-order thoughts] and each HOT represents
its target as belonging to the individual who also thinks the HOT in
question. (Rosenthal 2002: 15)

Across a range of such self-ascriptions, one develops a sense of being
their common subject. However, this sense could be wrong. The
experiences thus ascribed, says Rosenthal, could be supported by or
located in a diversity of subjects. It is because of this possibility
that Rosenthal asserts that all we have is a sense of consciousness
being unified.

Even when theorists such as Hume and Rosenthal deny that consciousness
is unified, sometimes unity of some sort still seems to be at work in
their models. Rosenthal says, for example, “A mental state is
conscious just in case it is accompanied by a … thought to the
effect that one is in the state in question” (2003:
325, our emphasis). If so, in addition to a HOT being about another
psychological state, it is also about oneself, the thing that has the
state (‘that one is in the state in
question’). Now, this consciousness of oneself is not
consciousness of any old object, it is consciousness of oneself,
oneself as the bearer of conscious states. But this is consciousness
of oneself as, to use Kant’s phrase, the single common subject
of one’s experience (1781/7, A350). If so, Rosenthal allows that
one kind of unified consciousness exists despite himself.

More recently, Nagel (1971), Davidson (1980), Dennett (1991, 1992),
O’Brien and Opie (1998), and Rosenthal (2003) have all urged in
one way or another that the mind’s unity has been overstated.
The point these people make is not just that the mind works mostly out
of the sight and often out of the control of consciousness. Virtually
everyone agrees on that and the point would in no way tell against
there being a real unity of some kind in the part that does enter
consciousness. Rather, they maintain that not even all conscious
states are unified with other conscious states.

Note that the claim here is not just that we are conscious of less of
the contents of our mind than we think, as Freud and the
psychoanalytic tradition argue. The claim is that our consciousness of
even many states of which we are conscious is not, or not fully,
unified. That we act against what we clearly know to be our own most
desired course of action or do things while telling ourselves that we
must avoid doing them are advanced as reasons for holding the view but
these are not obvious examples of lack of unity. A change blindness
experiment might offer a more substantial example.

In this experiment, a subject sits in front of a computer screen
wearing an eye tracker visor. There is a paragraph of text on the
screen. The subject is asked to read the text. When subject’s
eyes are focused on a particular word or phrase, the bit of text is as
it should be. However, everything around the word or phrase at
fixation, everything above, below and on both sides, beyond about 5
degrees of arc is chaotic: gibberish, out of order words, shapes
vaguely like words but obviously not words, and so on. Each time the
subject’s eyes shift to a new bit of text, it pops into correct
form just before the eyes get to it and the word or phrase just left
goes strange. What is remarkable about this experiment is that, while
to observers all appears chaotic except for a succession of words or
phrases—they are marching to a different saccadic
drummer—the subject has no consciousness that anything in the
paragraph is ever out of the ordinary. If the subject is one of those
people who automatically reads any text in front of her, often she has
no idea that the experiment has even started!

What this experiment shows, Dennett (1991: 361–362) thinks, is
that we take only a tiny, central bit of the scene in front of us
fully into unified consciousness. Yet subjects are conscious of the
rest of the screen in some ways. They are aware of movement, for
example. If so, this experiment would be an example of conscious
states that are not fully taken up into unified consciousness. We
would have unified consciousness of fewer of our conscious states than
has been thought.

Note that the argument just examined is not an argument that unified
consciousness does not exist. Even if we have conscious states that
are not unified or fully unified in consciousness, the most that that
could force us to do would be to shrink the range of states over which
consciousness is unified or fully unified. From the fact that not all
of one’s conscious contents are unified, it does not follow that
none is.

Indeed, those who hold that the extent to which consciousness is
unified has been overstated owe us an account of what has
been overstated. When theorists claim that a some conscious states are
not in unified consciousness, we should ask: Not unified with what?
One plausible answer would be: The unified conscious mind. Here is one
way to view the matter. Once upon a time, some theorists held that all
conscious states are unified and indeed that all mental states are
conscious. As we saw, Brentano is an example. (We will return to this
view in the next Section.) The main difference between this
pre-twentieth century vision of unified consciousness as ranging over
everything in the mind and view we just examined is that on the latter
view, less of consciousness is unified than the earlier view held.

Dennett is interesting in this regard. As we saw, he can plausibly be
read as rejecting the traditional picture of unified consciousness.
Yet he can still invoke unity. He says, “What is it like to be
an ant colony? Nothing, I submit … What is it like to be a
brace of oxen? Nothing (even if it is like something to be a single
ox)” (Dennett 2005). Why is the answer nothing? In such cases,
“there is no functional unity …—no unity to
distinguish an I from a we” (Dennett 2005).

To sum up the discussion of scepticism about unified consciousness so
far, the argument that the unity of consciousness is real, indeed is a
central feature of our kind of mind, seems to be strong.

3.2 Consciousness Must be Unified

That said, there are theorists who maintain not only that some
conscious states in a subject are unified but that all conscious
states must be unified. Bayne and Chalmers (2003: 24; Bayne
2010: ch. 1.4) call this the unity thesis: necessarily, any
set of conscious states of a subject at a time is unified. As we saw,
Brentano probably held this view. Hill (1991) does, too. Scepticism
about this view would be weaker than either of the two kinds of
scepticism about unified consciousness that we just examined.

Bayne and Chalmers express sympathy for the thesis on the grounds
that,

It is difficult or impossible to imagine a subject having two
phenomenal states simultaneously, without there being a conjoint
phenomenology for both states. (2003: 37)

Merely having phenomenal states might seem too little but
Bayne and Chalmers are talking about phenomenal states where, for
them, to have the state is for the state to be like something. If we
recast to make this element explicit, we get a claim of some real
intuitive appeal: If A is like something to S and
B is like something to S, it must be the case that the
combination, A and B, is like something to S.
Interestingly, Kant seems to have believed something similar:
“[Experiences] can represent something to me only insofar as
they belong with all others to one consciousness” (A116).
A and B having conjoint phenomenology is exactly what
unity consists in, according to Bayne and Chalmers. Put this way, the
unity thesis has some real appeal.

Are there reasons to be sceptical of the unity thesis, presumptive
counter-examples say? Spelled out as we have spelled it out, we do not
know of any. Bayne and Chalmers consider brain bisection cases to be
putative counter-examples because, on some concepts of the subject of
experience, we can think of there still being one subject in these
cases even though not all the conscious states are unified. There are
at least three ways to respond. The simplest is just to deny that
there is one subject, at least for the period of the split. A second
would be to note that, however one counts subjects during the period
of the split, there is evidence that many conscious experiences in
that body are not like anything to some subject. If so, the apparent
lack of conjoint consciousness of them will not be a problem.
A third (advocated by Bayne & Chalmers 2003: 38–9; Bayne
2010: ch. 9) would be to urge that while there is clearly a breach in
the unity of access consciousness (access to information for purposes
of belief formation, behavioural control, and so on) during the period
of the split, phenomenal unity may still extend across all the
conscious experience. We will discuss the third response in more
detail in
Section 4.2
below.

The unity thesis is a very strong thesis. Theorists could hold both
that consciousness is unified and that this unity is important and yet
deny that the unity thesis is true.

4. Disorders of Unified Consciousness

One of the most interesting ways to study psychological phenomena is
to see what happens when they take an abnormal form or break down.
Phenomena that look simple and seamless when functioning smoothly
often reveal all sorts of structure when they begin to malfunction.
The unity of consciousness can be damaged and/or distorted in both
naturally-occurring and experimental situations. What can we learn
from these cases?

As a rough grouping tool, we will treat unified consciousness as
breaking down in ways that fit a two-by-two matrix. The cases we will
consider can be grouped according to whether unified consciousness
appears to continue but in an unusual form, one instance of it
splitting into two for example, and situations in which it appears to
deteriorate more severely, to the point where it may even be said to
have been destroyed. And we will find examples of both kinds that
occur at a time and across time. Indeed, some of the disorders that we
will examine are both at a time and across time.

At a time, situations of disorder in which unified consciousness is
retained may take a number of forms. There are cases where
consciousness seems to split within one brain and body. The much
discussed commissurotomies (brain bisection operations) are the best
known example. (Across time, the closest analogue may be dissociative
identity disorder, about which more shortly.) There may be cases in
which a single occurrence of unified consciousness spans two bodies;
some have said this about some mirror twins. And there are cases in
which the array of phenomena over which unified consciousness ranges
becomes strangely circumscribed: hemi-neglect and anosognosia are two
examples. (We will describe all these phenomena in more detail below.)
What is interesting for our purposes is that in all these kinds of
cases, those in which unified conscious seems to split, those (if any)
in which it seems to span two bodies, and those in which its range
seems to shrink, the unity itself seem to be intact. It has not been
destroyed or even damaged. It is just housed in an unusual way.

The cases just introduced contrast with situations in which we have
just one instance of consciousness of some kind ranging over the usual
phenomena or some of them, but where the unity, to dramatize a bit,
appears to have shattered, not split or expanded or shrunk. In these
cases, the unity of consciousness is not just unusually housed. It has
been seriously compromised.

4.1 Some Disorders

4.1.1 Brain Bisection Operations

No medical procedure to do with consciousness has received as much
philosophical attention in recent times as commissurotomies, more
commonly known as brain bisection operations. Nagel (1971) was perhaps
the first philosopher to write on them; his paper continues to be
influential. Since then, Puccetti (1973, 1981), Marks (1981), Hirsch
(1991), Lockwood (1989), Hurley (1998), Bayne (2008, 2010), Schechter
(2010) and many, many other philosophers have written on these
operations. Indeed, the strange results of these operations in certain
controlled conditions was one of the things that brought the unity of
consciousness back onto the cognitive research agenda.

In these operations, the corpus callosum is cut. The corpus callosum
is a large strand of about 200,000,000 neurons running from one
hemisphere to the other. When present, it is the chief channel of
communication between the hemispheres. These operations, done mainly
in the 1960s but recently reintroduced in a somewhat modified form,
are a last-ditch effort to control certain kinds of severe epilepsy by
stopping the spread of seizures from one lobe of the cerebral cortex
to the other. For details, see Sperry (1984), Zaidel et al. (1993), or
Gazzaniga (2000).

In normal life, patients show little effect of the operation. In
particular, their consciousness of their world and themselves appears
to remain as unified as it was prior to the operation. How this can be
has puzzled a lot of people (Hurley 1998). Even more interesting for
our purposes, however, is that, under certain laboratory conditions,
these patients seem to behave as though two ‘centres of
consciousness’ have been created in them. The original unity
seems to be gone and two centres of unified consciousness seem to have
replaced it, each associated with one of the two cerebral
hemispheres.

Here are a couple of examples of the kinds of behaviour that prompt
that assessment. The human retina is split vertically in such a way
that the left half of each retina is primarily hooked up to the left
hemisphere of the brain and the right half of each retina is primarily
hooked up to the right hemisphere of the brain. Now suppose that we
flash the word TAXABLE on a screen in front of a brain bisected
patient in such a way that the letters TAX hit the left side of the
retina, the letters ABLE the right side, and we put measures in place
to ensure that the information hitting each half of the retina goes
only to one lobe and is not fed to the other. If such a patient is
asked what word is being shown, the mouth, controlled usually by the
left hemisphere, will say TAX while the hand controlled by the
hemisphere that does not control the mouth (usually the left hand and
the right hemisphere) will write ABLE. Or, if the hemisphere that
controls a hand (usually the left hand) but not speech is asked to do
arithmetic in a way that does not penetrate to the hemisphere that
controls speech and the hands are shielded from the eyes, the mouth
will insist that it is not doing arithmetic, has not even thought of
arithmetic today, and so on—while the appropriate hand is busily
doing arithmetic!

Recently, this standard assessment has been challenged. In his
important 2010 book and other publications, Tim Bayne suggests that
there are not two centres of consciousness. There is just one. What
explains the appearance of duality is that this single centre of
consciousness switches in the material of which it is consciousness
from one hemisphere to the other. He calls this the switch model
(2010: chs 8.4 and 9.5). To so much as get this approach off the
ground, it is crucial that that there be little or no evidence of a
centre of consciousness that is conscious of, say, A, B,
and C but not D, E, and F and at the same
time of a centre of consciousness that is conscious of D,
E, and F but not A, B, C. What we
just called the standard assessment starts from a belief that there is
lots of such evidence. Bayne disagrees. We will return to
Bayne’s view in
Section 4.2.

Because brain bisection operations have attracted so much attention
outside of psychiatry and neurology, we have included references to
some of the more important writings. We will not do so for the rest of
the disorders we will introduce. For fuller accounts and references,
consult a general textbook of psychiatry. Here we are interested in
them only for the vicissitudes of unified consciousness that they
display, or might be thought to display.

4.1.2 Mirror Twins

In addition to cases in which one body may have two centres of unified
consciousness, there are cases in which one centre of unified
consciousness may span two bodies. Mirror twins sometimes have such an
appearance. Mirror twins are biologically identical twins where each
body mirrors the other. They dress exactly the same, finish each
other’s sentences, do tasks together whenever possible (if an
egg is being cooked, both right hands will hold the handle of the
frying pan), and so on. Brain bisection cases are putative examples of
the number of centres of consciousness not lining up with the number
of brains and bodies from one direction. Mirror twins might be an
example from the opposite direction. In connection with unified
consciousness, this possibility is interesting.

The best-known case is the case of Greta and Freda Chaplin, who came
to light in the U.K. in the 1970s. Two bodies were involved but the
bodies acted in ways that would have been compatible, at least, with a
single instance of unified consciousness spanning them. Each body
could finish complicated and unpredictable sentences started by the
other. The two did everything they could together. When separated by
more than a few metres, they complained bitterly, each body reporting
that it felt like a part of itself was being ripped out. And so on. No
professional discussion of the case has been found, unfortunately, but
it was widely reported in the press at the time, for example in
Time, Apr. 6, 1981.

4.1.3 Hemi-neglect and Anosognosia

In hemi-neglect, one loses all sense of one side of one’s body
or sometimes one half (divided vertically) of everything spatial in
one’s experience. Whatever is going on in hemi-neglect, unified
consciousness seems to remain. It is just that its ‘range’
has been bizarrely circumscribed. It encompasses an experience of only
half the body or half of objects seen, not of the whole body or whole
objects. Where we expect perception and proprioception of the whole
body and whole objects, these patients perceive and propriocept only
one-half of the body and/or objects in general. So hemi-neglect is
another phenomenon in which there may be a major change in the
phenomena over which unified consciousness without unified
consciousness itself being degraded.

Another phenomena in which unified consciousness seems to remain but
with a bizarrely circumscribed range is anosognosia. In this
condition, a person who has suffered loss of function (often as a
result of a stroke) is unaware of the deficits (Gennaro 2015a). Thus,
a person now blind will insist that she can see—and will stumble
about in a room bumping into things. A person whose limbs are now
paralysed will insist that his limbs are moving—and will become
furious when family and caregivers say that they are not. And so on.

4.1.4 Dissociative Identity Disorder

Another candidate phenomenon is what used to be called Multiple
Personality Disorder, now, more neutrally, Dissociative Identity
Disorder (DID). Everything about this phenomenon is controversial,
including whether there is any real multiplicity of
consciousness at all (Hacking 1995; Dennett 1998a). DID can take
two forms. The more common form is often described as the units
(persons, personalities, sides of a single personality, or whatever
one decides to call them) ‘taking turns’, usually with
pronounced changes in personality. When one is active, the other(s)
usually is(are) not. Here the most prominent symptom is usually
strange memory gaps (amnesias) in each ‘unit’ for periods
when the body in question was clearly conscious and active. In the
other, less common form, both ‘units’ are present at the
same time. Here, for example, the unit in control of speech will
report that another ‘person’ inside her is, say, giving
her orders, these orders being experienced not from the standpoint of
giving them but as coming from another person. This form of DID is
called the co-conscious form in the literature but the term names
something very different here from what James, Parfit and the like had
in mind when they used the term. Among other things, me and the
‘little person inside me’ are not unified in one
consciousness. In fact, sometimes the dissociation in both forms of
DID is behaviourally as complete as it is in brain bisection patients
in the lab.

4.1.5 Schizophrenia

In some particularly severe forms of schizophrenia, the victim seems
to lose the ability to have an integrated, interrelated experience of
his or her world and self altogether. The person speaks in ‘word
salads’ that never get anywhere, indeed sometimes never become
complete sentences. The person is unable to put together perceptions,
beliefs and motives into even simple plans of action or act on such
plans if formed, even plans to obtain sustenance, tend to bodily
needs, escape painful irritants, and so on. Here, it is plausible to
suggest that the unity of consciousness has shattered rather than
split. The behaviour of these people seems to express what we might
call mere experience-fragments, the contents of which are so narrow
and unintegrated that the subject is unable to cope with daily life
and interact with others in the ways that, for example, split brain
subjects can.

4.1.6 Dysexecutive Syndrome

In schizophrenia of the severe sort just described, the shattering of
consciousness is part of a general breakdown or deformation of mental
functioning: affect, desire, belief, even memory all suffer massive
distortions. In another kind of case, the normal unity of
consciousness seems to be just as absent but there does not seem to be
the same sort of general cognitive or affective disturbance. This is
true of what some researchers call dysexecutive syndrome (Dawson,
1998: 215, for example). What indicates breakdown in the unity of
consciousness is that these subjects are unable to consider two things
together, even things directly related to one another. For example,
such people cannot figure out whether a piece of a puzzle fits into a
certain place even when the piece and the puzzle are both clearly
visible and the piece obviously fits. They cannot crack an egg into a
pan. And so on.

Trevarthen (1984) reports a similar syndrome in a few patients. In the
cases he reports, commissurotomy patients are conscious of some object
seen in the right side of the visual field by the left hemisphere
(controlled so that the information is received by only that
hemisphere) until an intention is formed to reach for it with the left
hand, controlled by the right hemisphere. Somehow the intention to
reach for it seems to obliterate consciousness of it in the hemisphere
that controls speech, presumably the left hemisphere. However, if the
object is slid over to the left visual field, then the
speech-controlling hemisphere reports that it can see the object
again—even though the object can now be seen only by the right
hemisphere and the left still controls speech!

4.1.7 Simultagnosia

A disorder presenting similar symptoms is simultagnosia or
Balint’s syndrome (Balint was an early 20th century German
neurologist). In this disorder, patients see only one object located
at one ‘place’ in the visual field at a time. Outside of a
few degrees of arc in the visual field, these patients say they see
nothing but an “undifferentiated mess” and seem to be
receiving no information about objects (Hardcastle 1997: 62).

What is common to dysexecutive disorder, Trevarthen’s cases, and
simultagnosia is that subjects seem not to be conscious of even two
objects in a single conscious state. They cannot, for example, compare
the objects (in Trevarthen’s cases, the object of a perception
with the object of an intention). Unlike commissurotomy cases, it is
not the case that a conscious experience of the second item exists
within another unified consciousness. If there is any experience of
the second item at all, it is not conscious. Rather than consciousness
being split into two discrete parcels, there is just one diminished
parcel. The rest of the conscious experiencing that is typical of
normal consciousness has disappeared.

There are of course many different theories about what is going on in
the conditions we have just sketched, severe schizophrenia,
dysexecutive syndrome, simultagnosia/Balint’s syndrome. Some
hold that the deficits are not in unified consciousness at all; they
are in the capacity to process perceptual information. On this view,
consciousness remains unified but patients can no longer can take in
what is happening. Here we will explore only the idea that the problem
is with unified consciousness. If it is even possible that this is
where the problem is, we can learn interesting things about the unify
of consciousness.

Many of the disorders that we have considered are fairly directly a
result of changes to the brain. Cognitive neuroscience is the study of
the relationship between the two. The best-known objects of such study
in connection with unified consciousness are brain bisection
operations (commissurotomies). Other neuroscientific studies relevant
to unified consciousness have examined blindsight (Weiskrantz 1986),
blindsight with visual agnosia (van Gulick 1994), hallucinations and
thought insertion (Stephens & Graham 2000), and archaic hand
syndrome and delusions of control in schizophrenia (Mylopoulos 2015).

4.2 Do the Conditions have a Common Structure?

Can a single structure account for what is going on in all these
phenomena? The Kantian taxonomy of forms of unified consciousness
considered in
Section 2.2
can give us a way into this issue.

As we said earlier, one natural way to think of the conditions just
sketched is to break them into two groups. In one group, however
drastic the change in unified consciousness, unified consciousness
remains in a largely complete form. Arguably, this group would include
brain bisection cases, hemi-neglect, anosognosia, and DID. In the
second group, schizophrenia of the severe kind we sketched,
dysexecutive disorder, and simultagnosia, it is more natural to think
that unified consciousness has been damaged or even destroyed.

Brain bisection cases first. On the standard way of conceptualizing
these cases, the key evidence for a duality of some kind is that there
appear to be situations in which whatever is conscious of some items
being experienced in the body in question is not conscious of other
items being experienced in that same body at the same time. We looked
at two examples of the phenomenon in
Section 4.1,
the word TAXABLE and the doing of arithmetic. With respect to these
experienced items, there is a significant and systematically
extendable situation in which to be conscious of some of these items
is not to be conscious of others of them in a single unified
consciousness where we would expect such consciousness. If so, brain
bisection patients fail to meet the conditions for unified
consciousness of contents. This seems to be what motivates the
judgment that these patients have two centres of consciousness.

Let us describe the case a bit more precisely. On the standard
account, a brain bisection patient could be conscious that both TAX
and ABLE are being seen. But nothing in the patient would be conscious
that both items are being seen on the basis of seeing them.
For at least one of the two items, one of the two centres of
consciousness could have only the same kind of behavioural and other
‘outside’ evidence that any observer of the situation
could have.

Indeed, the apparent split runs still deeper. Between the two
hemispheres there seems to be a split in unified consciousness of
experiencing, too: Consciousness of doing some experiencing
goes with lack of consciousness of doing other experiencing that is
going on in the same body. To use a useful metaphor first coined, so
far as we know, by Shoemaker, something is not conscious of acts of
experiencing going on in its body ‘from the inside’, i.e.,
on the basis of doing them, while something else is conscious
of them on this basis. There seems to be a split in unified
consciousness of self, too: Consciousness of oneself as
subject on the basis of doing acts of experiencing in that body goes
with lack of consciousness of oneself as subject on the basis of other
acts of experiencing being done in that body. If so, for many
conscious states in these patients, there two instances of
joint consciousness (section 2.1.3),
not the normal one.

To be sure, this assessment is not universally accepted. As we saw,
Bayne and Chalmers (2003) and Bayne (2010: ch. 9) urge that while
access unity is split in these patients, phenomenal unity need not be.
Why not? Because there may be no duality of unified consciousness at
any given time. Rather, a single instance of unified consciousness may
be switching back and forth between the material in the two
hemispheres. As Schechter (2012) has urged, evidence of a simultaneous
duality of consciousness would be a major problem for this
approach. We will not attempt to assess the relevant evidence here
(see Brook 2015 for further discussion). Even if Bayne is right, that
should not affect the prospects of a unified account of disorders of
unified consciousness—what is going on in brain bisection cases
would not be a disorder of unified consciousness!

Next, dissociative identity disorder (DID). (We will take up
hemi-neglect and anosognosia shortly.) In cases of DID, a central
feature is either some pattern of reciprocal amnesia or a strong sense
that another is inside (and yet still separate). This again seems to
be a situation in which being conscious of some experienced objects by
having the experience goes with not being conscious of others in the
same body in the same way. The main difference is that the breach
seems to be at a time in brain bisection cases, but can be either
across time or at a time in DID cases. If so, the breakdown in unity
will again consist in breakdowns of joint consciousness. The amnesia
in diachronic DID has this character, clearly (and, except for the
amnesia, is in line with Bayne’s (2010) switch model), but so
does synchronic DID. The person and the ‘little person
inside’ have no access from the inside to each other, so no
common unified consciousness of experiencing, and the objects of their
experience are not unified, so no common unified consciousness of
contents.

Now the second group. For severe schizophrenia, dysexecutive syndrome,
and simultagnosia/Balint’s syndrome—the cases where
consciousness seems to be more shattered than split—the
distinction of Kant’s that we introduced in
Section 2.2
between two kinds of synthesis is useful. As we saw, Kant
distinguishes between the synthesis that gives us consciousness of
individual objects (Tye’s object unity) and the synthesis that
gives us consciousness of a number of objects at the same time.

This distinction seems to shed some interesting light on the three
phenomena. The evidence suggests that the first kind of synthesis
continues to be available to dysexecutive and simultagnosia patients:
they continue to be conscious of individual objects, events, etc. The
damage seems to be with respect to the second kind, being conscious of
multiple objects in a single act of consciousness. These people seem
to achieve some measure of unified focal attention with respect to
individual objects but unified consciousness of multiple objects is
either restricted or missing.

With the severe forms of schizophrenia that we sketched, patients may
lack even the ability to perform the first kind of synthesis. In a
different jargon, these people may lack even the capacity for object
constancy.

On this analysis, hemi-neglect and anosognosia are a bit different
from the other conditions. Here there is no apparent breach of joint
consciousness. Neither a split nor a breakdown in unified
consciousness is evident. Rather, in both conditions, there appears to
be a shrinking of the array of phenomena over which otherwise intact
joint consciousness can range. Half of one’s body and/or half of
all perceived objects are excluded in the first condition, the actual
situation with respect to sight, limbs, etc., in the second.

To sum up, it appears that there is some prospect of placing all the
conditions we have considered within a single structure of (Kantian)
distinctions, first between unified consciousness of contents and
unified consciousness of experiencing, then between conscious
experience of individual objects and unified conscious experience of
multiple objects. Thought insertion might pose a problem for this
scheme. Patients experience thought insertion when they believe that
some of their thoughts, experiences, emotions, and so on are not their
own and have been ‘inserted’ into them from without.
People suffering it are still aware of the ‘alien’
thoughts from the point of view of having them. The deficit is in
something to do with thinking of these thoughts as unified with the
rest of conscious life. Normally, we think of ourselves as the subject
and agent of all our experiences. In thought insertion, the victim
does not appear to himself to be either subject or agent of some of
the experiences of which he is in fact the subject and agent
(Mylopoulos 2015).

5. Unity Across Time

Unified consciousness at a given time (synchronic unity) has mainly
been our topic so far. We now turn, more briefly, to unified
consciousness over time (diachronic unity). As was noted as long ago
as Kant, unity across time is required even for such rudimentary
mental operations as counting (1781: A103); indeed, unity across time
is crucial for virtually all cognition of any complexity. Now,
unification in consciousness might not be the only way to
unite earlier cognitive states (earlier thoughts, earlier experiences)
with current ones but it is certainly a central way and the one best
known to us.

5.1 Retention and Memory

In its synchronic form, we have suggested that a natural way to think
of unified consciousness is in terms of joint consciousness.
Diachronically, unified consciousness has an additional feature; it
requires retention over time, specifically, retention of earlier
experienced contents as one experienced them. What the retention
crucial to diachronic unity consists in is a matter of some interest.
It is tempting to assume that it is a kind of memory. However, as
Husserl already told us, there is reason to be sceptical of this
approach. There is a difference between experiencing a
succession from time 1 to time 2 and merely remembering
experiencing what happened at time 1 while experiencing something at
time 2. Dainton captures Husserl’s point by noting the
difference between “immediate and represented
experience—remembering or imagining hearing a tone is not the
same as directly experiencing the tone” (Dainton 2005: 155;
Dainton cites Husserl 1928).

Kelly (2005) raises a similar question. Suppose that one is listening
to a melody. It has five notes and the final note is just being
played. If one simply recollected the earlier notes, one should
experience a chord, not five notes spread out and related to one
another in time. Somehow, the earlier notes come
‘date-stamped’ but still available to be integrated with
the current experience in a single, temporally-extended, unified
experience. Whatever this process is like, it is clearly vital to our
kind of unified consciousness. Without it, one could not hear any
sequence as a sequence or so much as read a simple sentence. Though
some theorists call this across-time process unity of consciousness, a
more distinctive name for it would be the continuity of
consciousness.

This sort of continuity of consciousness can span very short durations
(such as the ‘specious present’). Even a seemingly simple,
current experience is in fact a continuous experience of more than one
instant, and must be if one is to hear a sound or perceive
(as opposed to remember) any temporally stretched phenomenon. How can
one have a unified conscious experience (not just a memory) of
duration?

Here again the debate that we mentioned earlier over whether a unified
conscious experience is one experience or an assembly of many
experiences rears its head. Dainton (2005: chs. 5–7) takes a
continuous, unified experience to include co-conscious experiences as
parts. Tye (2003: ch. 4) urges instead that a diachronically unified
experience has multiple contents but no experiences as parts.

5.2 Unity and Personal Identity

In the history of European philosophy at least since Locke, diachronic
unified consciousness has been closely linked to personal identity in
the philosopher’s sense, i.e., continuing to be a single person,
one and the same person, across time. (The point of the restriction to
philosophy is that clinical psychologists use the term quite
differently, as the name for certain aspects of personality and
‘self-conception’.) Whatever may be true of the kind of
diachronic unity we just discussed, the kind of diachronic unity
associated with personal identity is clearly a kind of memory,
specifically, a kind of autobiographical memory. At least since Locke,
philosophers have argued that as far back as unified consciousness
via the right kind of autobiographical memory extends, there
extends the person, one and the same person over all this time. The
right kind of autobiographical memory is memory of the having,
feeling, or doing of earlier experiences, emotions, actions, and so
on. As Locke has it, being the same person just is having the
‘same consciousness’. We must be careful here.
There is lots of autobiographical memory that is not memory from the
point of view of experiencing. A person can remember that so-and-so
happened to her without remembering the event, the experience of it,
or anything else ‘from the inside’, to use
Shoemaker’s useful metaphor again. Memory theorists’
standard categories are not fine-grained enough for our purposes here.

Some important philosophers have urged that memory-carried diachronic
unity is not sufficient for being one person over time. Kant, for
example, argued for a dissociation here, in his famous critique of the
third paralogism. In Kant’s view, continuity sufficient to
“retain the thought of the previous subject and so hand it over
to the subsequent subject” (1781: A363), continuity sufficient
therefore for diachronic unity of consciousness, is quite compatible
with the ‘retained thoughts’ being passed from one subject
to another, compatible therefore with an utter absence of personal
identity. If so, diachronic unity is not sufficient for personal
identity (Brook 1994: ch. 8). (Note: Locke and Kant may be less far
apart than this brief discussion would suggest. We are merely using
them to illustrate the two positions, not discussing either of them
fully.)

Phenomena relevant to identity in things other than persons can be a
matter of degree. This is well illustrated by the famous ship of
Theseus. Suppose that over the years, a certain ship was rebuilt,
board by board, until every bit of it has been replaced. Is the ship
at the end of the process the ship that started the process? Now
suppose that we take all those rotten, replaced boards and reassemble
them into a ship. Is this ship the original ship? It seems
that there is no determinate answer to these questions. Say what you
like, and what you like may vary depending on whether you are an
insurance adjuster or a history buff. Many philosophers have insisted
that such indeterminacy can never be the case for persons. Identity in
persons is always completely unambiguous, not something that could
ever be a matter of degree (Bishop Joseph Butler [1736] is a
well-known example).

Brain bisection cases (described in
Section 4.1)
in which, some urge, unified consciousness splits in two may be
relevant here (Parfit 1971, 1984). As Parfit argues, the possibility
of persons (or at any rate minds) splitting and re-fusing puts real
pressure on intuitions about our specialness. Perhaps the continuity
of persons can be just as tangled and just as much a matter of degree
as the continuity of any other middle-sized object.

Two final comments. Nagel (1971) argues that there can be
indeterminacy in synchronic unity, too (see
Section 6). One can sympathize
with Parfit about diachronic unity and yet have reservations about
Nagel on synchronic unity. Likewise, one should distinguish the
question of whether diachronic unity can be intransitive from the
question discussed in
Section 6
of whether synchronic unity can be intransitive.

6. Two Philosophical Questions

Philosophers have made some fairly exotic claims about brain bisection
cases and related conditions. Here we will consider two of them.

6.1 No Whole Number of Centres of Consciousness

The first is a claim that in brain bisection patients, there is no
whole number of persons. So far, we have talked as though in brain
bisection we always end up with some clear number of instances of
unified consciousness. Nagel (1971), one of the early philosophers to
write about these cases, rejects that view. For him, there is no whole
number of ‘centres of consciousness’ in brain bisection
patients: there is too much unity (for example in life outside the
laboratory and even in behaviour within) to say “two”, yet
too much separation in the specially contrived laboratory situations
to say “one”.

Not being happy with so counterintuitive a result, philosophers have
responded. A response favoured by many is this. For any
precise ‘one or two?’ question, there will be a
precise answer. Behavioural control system? One. Groups of experienced
objects unified in consciousness? Two. And so on. If so, while the
one’s and the two’s wouldn’t line up as tidily as
they do most of the time in people who have not had this operation, it
is not obvious that the mixed answers support Nagel’s conclusion
that there may be no whole number of centres of consciousness in these
patients.

6.2 Partial Unity is Possible

The ‘in some respects one, in some respects two’
possibility at the centre of Nagel’s analysis is related to a
question about transitivity. Is it possible, for a given instance of
experienced objects p, q, and r, for there to be
unified consciousness of p and q, unified consciousness
of q and r, but no unified consciousness of p and
r? (The parallel in brain bisection cases? Call the
‘centres of consciousness’ in the two cerebral lobes
A and C, the older unilateral brain below them B.
Is it possible for a mental state in A to be unified in
consciousness with one in B, one in B with one in
C, and yet the state in A not to be unified with the
state in C?)

Hurley (1998) examines this question in detail, as does Bayne (2010:
ch. 9.4). Hurley comes at the question of transitivity by considering
some results reported by Sergent (1990). Since Sergent’s work
has not been replicable, let us look instead at how the research by
Trevarthen that we mentioned in
Section 4.1.6
throws up the same issue. In this research, as we said, brain
bisection patients under certain conditions are conscious of some
object seen by, say, the right hemisphere until the left hand, which
is controlled by the right hemisphere, reaches for it. Somehow the act
of reaching for it seems to obliterate the consciousness of it. Very
strange—how can something pop into and disappear from unified
consciousness in this way? This question leads Hurley to the notion of
partial unity. Could two centres of consciousness, A
and C, though not unified in consciousness with one another,
nonetheless both be unified with some third thing, in this case the
volitional system B (the system of intentions, desires, etc.)?
If so, ‘being unified with’ is not a transitive
relationship—A could be unified with B, and
C could be unified with B, without A being
unified with C. This idea is puzzling enough. Even more
puzzling is how activation of the system B, with which both
A and C are unified, could result in the loss of
consciousness in A and/or C of an object aimed at by
B.

Hurley never pronounces on the possibility of partial unity. Instead,
she argues that none of the cases suspected of displaying it really
do. She accepts that intention can obliterate consciousness—but
then distinguishes time periods (1998: 216). In Trevarthen’s
cases, for example, the situation with respect to unity is clear at
any given moment—one either is or is not conscious of the
object. The picture over time does not conform to our usual
expectations for diachronic singularity or transitivity of
unity—but that is simply an artefact of the cases, not a
problem.

Hurley considers another class of cases, what she calls Marcel’s
(1994) cases. Here subjects are asked to report the appearance of some
item in consciousness in three ways at the same time—say, by
blinking, pushing a button, and saying, ‘I see it’.
Remarkably, in different trials each of these three acts are done
without doing the other two. And the question is, What does this imply
for unified consciousness? In a case in which the subject pushes the
button but neither blinks nor says anything, for example, is the
hand-controller aware of the object while the blink-controller and the
speech-controller are not? How could the conscious system become
fragmented in such a way?

Hurley’s suggestion? They can’t. What induces the
appearance of incoherence about unity is the short time scale. Suppose
that it takes some time to achieve unified consciousness, perhaps
because some complex feedback processes are involved. If so, then
Marcel’s cases have not got to a stable situation with respect
to unity. The subjects were not given enough time (1998: 216).

Is partial unity possible? To date, this remains an unanswered
question, though Bayne (2010: 209) leans towards a negative
answer.

Hurley discusses more aspects of the unity of consciousness than
partial unity. She argues, for example, that there is a normative
dimension to unified consciousness—conscious states have to
cohere semantically for unified consciousness to result (we will
return to this issue in
Section 7.3).
She discusses most of the kinds of breakdown phenomena that we
considered earlier, exploring the implications of a wide range of
‘experiments of nature’ and laboratory experiments for the
presence or absence of unified consciousness. In particular, she
considers acallosal people (people born without a corpus callosum).
Even though the corpus callosum, when present, is the chief channel of
communication between the hemispheres, acallosal people show all the
behavioural signs of having fully unified consciousness. If so, then
the neurological and behavioural basis of unified consciousness would
be very different in different people. (We will return to this last
topic in
Section 8.)

7. Theories of Unity

In the literature, there is quite a range of theoretical claims about
unified consciousness. We have looked at some of them. The first group
concerned diverging models: the subsumption, co-consciousness, single
phenomenal content, and joint consciousness models and related
taxonomies. Then we looked at the unity thesis, claims about limits to
and disorders of unified consciousness, claims about unity over time,
and claims that there need not be a whole number of centres of
consciousness and that partial unity is possible. In
Section 2,
we said that we’d return to the issue of whether unified
experiences has or does not have experiential parts (EP vs. NEP), the
issue that underlays subsumption. EP and NEP are the topics of
Sections
7.1
and
7.2.
Near the end of
Section 4,
we said that we would return to the claim that unified consciousness
requires links among conscious contents. We will take up this issues
in
Section 7.3.
Finally for this Section, we will examine a claim that we have not
discussed so far, what is usually called the co-ownership thesis
(Section 7.4)

7.1 The Experiential Parts Theory

How is unified conscious experience structured? As we mentioned in
Section 2.1,
two incompatible models have some currency at the moment. On the
experiential parts view (EP), unified conscious experience includes
simpler experiences as parts or something like parts; unified
consciousness has a mereological aspect. On this view, when I have a
unified experience of a pain and a noise, this unified experience
includes an experience of just the pain, and an experience of just the
noise. These simpler experiences are the relata of unified
consciousness; they are joined as parts of the unified experience of
the pain and noise together. Experiences a and b are
united in a third experience, c, which is their joint
occurrence. On the no experiential parts (NEP) account, the conscious
mental act through which diverse contents are presented does not have
other conscious states, experiences, as parts. On the first view, when
I have unified consciousness of experiencing, I am conscious of many
experiences. On the second view, I am conscious of just one
experience.

To clarify the two, let us use the notation ‘\(E(o_1)\)’
for an experience that is the conscious experience of just the
intentional object \(o_{1}\). A conscious experience of just \(o_{2}\)
is \(E(o_{2})\). What is the nature of an experience that takes the
bigger content in which \(o_{1}\) and \(o_{2}\) are presented
together? On NEP, it has the structure of \(E(o_{1}, o_{2})\), where
this introduces a single experience that has both contents as its
object. To be conscious of \(o_{1}\) by means of this experience is to
be conscious of it with \(o_{2}\). (See the concept of joint
consciousness introduced in
Section 2.1.3.)
According to NEP, this is what the subject’s conscious unity at
the time amounts to (if we oversimplify by supposing her to be
conscious of nothing but \(o_{1}\) and \(o_{2}\)). No
‘smaller’ or simpler conscious states figure as parts.
This experience might be realized in a brain state that has parts, but
these parts are not further conscious states. By contrast, in EP
\(E(o_{1})\) and \(E(o_{2})\) persist as parts of an encompassing
experience by means of which one is conscious of \(o_{1}\) and
\(o_{2}\) together and unified consciousness would have the structure
\(E(E(o_{1}) E(o_{2}))\).

Proponents of EP include Lockwood, Dainton, Shoemaker, and Bayne and
Chalmers, and Bayne by himself (in his 2010 book). As we saw in
Section 2.2,
theorists as otherwise different as Dainton, Lockwood, and Shoemaker
use the term ‘co-consciousness’ as the name for the
relationship that ties the experiential parts together. Not only are
most versions of EP built on some notion of co-consciousness; most
notions of co-consciousness assume EP and are incompatible with NEP.

EP faces a difficulty. James describes the problem in his example of
the twelve-word sentence. Suppose each word in the sentence is known
by just one of twelve people. It is hard to see, James says, how these
twelve thoughts could be combined to yield a unified consciousness of
the sentence. As he says,

Take a sentence of a dozen words, take twelve men, and to each one
word. Then stand the men in a row or jam them in a bunch, and let each
think of his word as intently as he will; nowhere will there be a
consciousness of the whole sentence. (James 1890: 160)

What EP needs is a way of combining experiences that does not simply
conjoin them into an experiential aggregate, for a mere combination of
experiences is not the experience of a combination. EP needs, then, a
way of putting together experiences that also puts together their
contents. Without any specification of how this combining of contents
is to be achieved, we are left with a mere aggregate of experiences,
each member of which is oblivious to the contents of the other states
in the aggregate. As James puts it, “Idea of a + idea of
b is not identical with idea of (\(a + b\))” (1890:
161).

Proponents of EP may reply that they never intended to give an
account of conscious unity. Instead, EP should be taken to
provide a description of the structure of unified conscious
states. In other words, it is a characterization, not an explanation,
of such states and how they are individuated. It may be held that the
only good way to individuate conscious states is on the basis of
contents. Hence, since one is aware of many contents via a conscious
state, that state must itself consist of several simpler conscious
states that have (somehow) come to be unified. At the very least,
then, those who advocate NEP owe us an alternative basis for
individuating conscious states. We will return to this issue in the
next section.

As we saw, the notion of a unified experience subsuming simpler
experiences is central to Bayne’s and Chalmers’ (2003; see
Bayne 2010) account of the unity of consciousness. This may appear to
be a version of EP and Tye treats it so (without using that label;
2003: 21). However, Bayne and Chalmers hedge their bet. While they do
speak of the encompassing conscious state as involving “at least
a conjunction of each of many more specific conscious states”
(2003: 27), and of a “complex phenomenal state and a simpler
state that is intuitively one of its ‘components’”
(2003: 40), they also caution that thinking here in terms of “a
mereological part/whole relation among phenomenal states” should
be regarded only as an “aid to intuition rather than as a
serious ontological proposal” (2003: 40). So how their view
stacks up with respect to EP is not entirely clear.

7.2 The No Experiential Parts Theory

Searle and Tye are leading current advocates of NEP. Searle ([2000]
2002: 56) ventures that “maybe it is wrong to think of
consciousness as made up of parts at all”. For one has a
“single, unified, conscious field containing visual, auditory,
and other aspects” and “there is no such thing as a
separate visual consciousness” ([2000] 2002: 55). Do
“visual experiences stand to the whole field of consciousness in
the part-whole relation?” ([2000] 2002: 54). No, says Searle
(though he may do some backsliding later in the article).

Tye offers a similar view, which he dubs the ‘one-experience
view’ (Tye 2003: ch. 1). Considering the polymodal nature of our
experience, he says, “There are not five different …
experiences somehow combined together to produce a new unified
experience”. Instead, “there is just one experience
here” (2003: 27).

Part of what is at stake in this dispute is how to individuate (how to
count) experiences. EP theorists think that experiences go one to an
object: if you experience two things, you have two experiences. Or
even finer: if you experience one thing in two ways, you have had two
experiences. NEP theorists hold that some experiences can be
individuated differently; a unified act of conscious experiencing is a
single experience, experientially non-composite, no matter how many
objects it has.

So what are the arguments for the two views like? Theorists who accept
EP usually just assert or even assume it (we saw some examples of this
in
Section 7.1).
The idea just seems intuitively plausible to its adherents and they
tend not to argue for it. NEP is not intuitively obvious—even
people such as Searle who advocate it can find themselves sliding into
EP—and its adherents do argue for it.

James was the first champion of NEP. He endorsed it in the course of
repudiating the ‘mind-stuff theory’, according to which
“our mental states are composite in structure, made up of
smaller states conjoined” (1890: 145). Against this James says
that, while our experience is complex, this complexity is not a matter
of there being several experiences (or
‘feelings’) present in an encompassing experience. This is
because “we cannot mix feelings as such, though we may mix the
objects we feel, and from their mixture get new
feelings” (1890: 157). If one’s experience appears to
become more complex, that is a matter of a single
experience’s content being more complex, and is not the
addition of more experiences (of the diverse contents).
Indeed, he says, “We cannot even … have two feelings in
mind at once” (1890: 157).

Here is how this is supposed to work. If we say that experiences
a and b are fused to form experience c, we should
treat ‘fused’ as referring to a process in which a
and b are superseded by c, not included in it. A
and b have been replaced by c, in which their contents
are connected, and they (a and b) no longer exist. As
James put it, contrasting the unified consciousness of the whole
alphabet with the several states involved in consciousness of each
letter taken singly,

It is safer … to treat the consciousness of the alphabet as a
twenty-seventh fact, the substitute and not the sum of the
twenty-six simpler consciousnesses. (1909: 189)

This view clearly avoids the problem of how to combine experiences
that faces EP.

Since James had a concept of co-consciousness and we have linked
co-consciousness closely to EP, we should say a word about his
concept. It is not the same as the concept that we find in Parfit,
Lockwood, Hurley, Shoemaker et al. For James, co-consciousness relates
only to a multiplicity of items of which one is conscious. In unified
conscious experience of them, there is no multiplicity of conscious
states to enter into a ‘co’-relationship. Indeed, the
contents are made co-conscious by being presented together in a
single, noncomposite experience.

We will close this discussion with two notes about the relationship of
the dispute between EP and NEP to the transparency thesis that we
discussed earlier. This is the thesis that we are not directly
conscious of our own experiences. (‘Transparent’ here
means that while I am conscious via conscious states, I am
not conscious of them. I ‘see through’ them, as
it were; hence ‘transparency’.) First, all claims for both
EP and NEP that we have considered would seem to go through even if
the transparency thesis were true. So transparency would not undermine
this debate. Secondly, even Tye (2003), who accepts both NEP and
transparency, also accepts that NEP does not require or
entail transparency. Rather, he seems to think that the
transparency thesis is true and, since it is true, this constrains
what could be unified in phenomenal unit. Accepting the constraint, he
has to say that, “Phenomenal unity is a relation between
qualities represented in experience, not between qualities
of experiences” (Tye 2003: 36), however unintuitive
this claim may be. Certainly there are approaches that can both accept
NEP and reject transparency. That is true, for example, of those who
hold that conscious states are self-representing states, states of
which one becomes conscious just by having them. These theorists will
hold that one can be directly conscious of one’s own
conscious states, yet they could still hold that states of unified
consciousness have an NEP structure.

7.3 The Internal Links Theory

Now we turn to the idea that unified consciousness of contents and
experiencing requires some kind of phenomenally evident relation among
the contents of the unified conscious state (in addition to the
contents being aspects of a single unified act of consciousness) and
some attempts to model this relationship. We might call this the
relational model of unified consciousness. It is a descendant
of Kant’s claim examined in
Section 1
that unified consciousness requires conceptual interconnectedness in
the objects of consciousness; as he put it,

all appearances stand in a thoroughgoing connection according to
necessary laws, and hence stand in a transcendental affinity
of which the empirical affinity is the mere consequence. (1781/1787:
A113–114)

Kant’s argument for this claim seems to have been that synthesis
of represented objects to produce a single complex object is a
necessary condition of consciousness of self as single common subject.
Without represented objects being tied together in a single complex
object, one might be aware of the subject of an individual
representation but one could not be aware of the subject of one such
representation as the subject of other such representations. Rather, I
should have “as many-coloured and diverse a self as I have
representations of which I am conscious ..”. (B134)—as are
in fact had by me, for I would not, of course, be aware that it was
me.

One recent expression of the idea is Hurley’s (1998) claim that
conscious states must satisfy a normative requirement if unified
consciousness is to result. Specifically, they must ‘cohere
semantically’. (It is not entirely clear that satisfying this
normative requirement requires that the contents of consciousness be
linked in the way that Kant urged.) Even more recently, Revonsuo
(2003) has urged that phenomenal contents must be situated in the same
‘phenomenal space’ in order to be unified, adding,
“I am inclined to treat phenomeno-spatiality as the basic
unifying feature of human consciousness”.

For theorists of this persuasion, these phenomenally evident spatial,
causal, etc., relations among contents explain why my perceptual
states typically present one coherent world in which, for instance, a
wall in front of me and starting at one end of my visual field will
continue across that field. Are such connections among the contents of
an experience necessary for their being presented together in
experience? Some recent theorists have argued that they are not (Bayne
2004; Brook 2005). One can have unified consciousness of a siren that
that one is hearing, an average grade that one is calculating, and a
fictitious landscape that one is visualizing. In what possible way
could items as diverse as these have to be connected to one another?
They are not even all in space or contiguous to one another and they
are certainly not causally interconnected, yet I can experience all
three in a single act of unified consciousness.

Doesn’t there have to be at least logical coherence among the
contents? As we just saw, Hurley (1998) makes this claim, as does
Baars (1988). Hurley, for example, argues that we cannot believe
mutually inconsistent things when we are conscious of both in a single
unified experience. Could the disconnect among unified conscious
states extend to them actually being inconsistent with one another?
Contrary to Hurley and Baars, the evidence suggests that it could.
Suppose that one sees a stick immersed in water as being bent but
feels it to be straight or knows that this is an illusion. Here,
one’s conscious perception that it is bent conflicts with
one’s conscious belief that it is not bent, yet these states are
unified in one consciousness (Bayne 2000). Tye (2003: 38) does not
consider this illusion to be an example of incoherence. He holds that
here touch corrects vision, making the stick appear to be straight,
and so belief renders the appearance mere appearance. That claim is
controversial. Bayne (2004: 227) discusses another example, inverting
spectacles, more plausibly. These glasses render one’s visual
contents inconsistent with one’s tactile contents, yet
consciousness of the visual contents remains unified with
consciousness of the tactile contents.

In fact, it would seem that there can be incompatibilities even within
a perceptual modality. Thus, Tye (2003: 38–39) notes that there
are pictures that depict impossible situations. He also discusses the
waterfall effect, in which, after staring at a waterfall for some
time, if one looks at the adjacent rock face, a portion of the rock
surface will appear to be moving—and not moving (relative to the
area around it; not everyone accepts that both elements are
simultaneously present in the waterfall illusion; see Crane 1988).
These examples suggest that we can have unified consciousness of
pretty much any collection of items whatsoever, no matter how they are
related to one another. Indeed, it is not easy to specify any relation
among unified contents or acts of experiencing (beyond their being
unified) that is required for them to be unified. If so,
Hurley’s (1998) suggestion that meeting a normative requirement
is necessary for unified consciousness, specifically, a requirement
that the experienced properties of things in unified consciousness
cohere semantically, is in trouble.

7.4 The Co-Ownership Theory

Finally, there is a model that holds that unity of conscious states
consists in their ownership by a single subject, Bayne’s (2004)
co-ownership. Is co-ownership meant to be necessary, or sufficient, or
both? The idea that unified conscious experience must be had by a
single subject could be trivially true, as it would be if the subject
at a time is just defined as a set of unified contents. For the thesis
to become interesting, advocates of it would have to offer a richer
conception of the subject than that. On such a conception, the claim
that experiences are had by the same subject would involve
their attribution to the same extra-phenomenal substrate or bearer of
experiences, one that can be individuated independently of what is to
be found in experience, and thus independently of the notion of a
unified field of conscious contents. Stated thus, the thesis would not
be trivial and may well state a necessary condition of unified
consciousness.

However, if co-ownership is necessary, is it also sufficient? It seems
that is is possible for items that are not unified with one
another to be simultaneously presented to one and the same subject.
This seems to happen, for example, in split brain cases, or fictional
variations thereof—e.g., Parfit’s example of a single
subject who is able simultaneously to try out two alternative
approaches to solving a math problem (1984: 246–248). This state
of affairs would seem to be a case of parallel but nonunified
sets of conscious states had by the same subject, in some
good sense of the term ‘same subject’. The same would seem
to hold of the contents of a subject who switches back and forth
between lobes who lacked memory of earlier contents. If so,
co-ownership would appear not be sufficient for unity. A
fortiori, co-ownership would appear not to capture what is
distinctive about unified consciousness. Put differently, if
there is a requirement here of any kind, more than co-ownership seems
to be needed. The subject must have a certain kind of relationship to
the material. As Kant put it, the material must be something to the
subject (A116)—and this requires a subject to whom things can be
something. Mere ownership by itself would appear to fall short (Brook
1994: 135–9, discusses these issues in connection with
Kant).

Thought insertion may pose another problem for the co-ownership
thesis. In thought insertion, the alien states are unquestionably
co-owned with the avowed ones but there seems to be less than full
unity of consciousness.

In
Section 5.2,
we examined Kant’s claim that instances of diachronic unified
consciousness are able to extend beyond one person (by any criterion
for being one person other than unified consciousness). If so,
diachronic unity is not sufficient for singleness of person. On the
standard account, brain bisection cases suggest roughly the reverse
claim about synchronic unity; where there seem to be two instances of
unity in one body, by many criteria there is just one person
(remember, there is no question about singleness of person outside the
laboratory). If so, singleness of person is not sufficient for
synchronic unity.

8. Neural Architecture of Unified Consciousness

We will conclude this article with a brief look at some philosophical
speculations about what the neural architecture of unified
consciousness might or must be like. One of the hottest issues in
current consciousness research is the issue of how brains achieve
consciousness and what parts of the brain are most involved in doing
do, what the ‘neural correlates of consciousness’ (NCC)
are. Any real insights into the NCCs of consciousness in general are
also likely to contain insights into the NCCs of unified
consciousness. This literature is now so vast that it would take a
whole additional article to discuss the topic. (Koch 2004 is an
excellent review of the empirical neuroscience and Chalmers 2000 is
the most extensive exploration of the conceptual issues to date.)
Because of space limitations, here we will restrict ourselves to three
of the most influential philosophical approaches to what the neural
architecture of consciousness might be like, those of Paul and
Patricia Churchland (see for example Paul Churchland 1995: 214),
Daniel Dennett (1991), and Susan Hurley (1998).

The Churchlands’ view flows from a radical picture of neural
architecture in general. They urge that the architecture of the
processes underlying cognition and consciousness consists not of
transformations of symbolically encoded representations, as most
philosophers have believed, but of something like vector
transformations in phase spaces. Thus on their view, neural correlates
of unified consciousness are nothing remotely symbol- or sentence-like.

Dennett articulates an even more radical view, on both unity and the
architecture of it. For him, unified consciousness of
‘self’ is simply a short-lasting ‘virtual
captain’ coming to be as a result of a small group of
information-parcels gaining temporary dominance in a struggle with
other such groups for control of such cognitive activities as
self-monitoring and self-reporting. We take these transient phenomena
to be more than they are because each of them is the ‘me’
of the moment and they are tied to earlier transient selves by the
special form of autobiographical memory identified earlier. If the
temporary coalition of conscious states that is winning at the moment
is what I am, is the self, each temporal chunk of ‘self’
is likely to be found in different parts of the brain from other such
chunks and there will be many NCCs of unified consciousness in many
different places.

If Dennett is right, there would be reason for scepticism about what
Hurley calls the isomorphism hypothesis. The isomorphism
hypothesis is the idea that a given kind of change in consciousness
will always reflect, even be the result of, a given kind of change in
the brain. Hurley is sceptical about it, too. One way in which
skepticism about this hypothesis arises in her work is via
consideration of acallosals (people born without a corpus callosum).
Even though the corpus callosum, when present, is the chief channel of
communication between the hemispheres, acallosal people show all the
behavioral signs of having fully unified consciousness. If so, it has
to be achieved by mechanisms such as cuing activity that are utterly
different from communication though a corpus callosum. And the same
can be true the opposite way. Different changes in consciousness can
go with the same changes to structure and function in the brain. And
if both these claims are correct, then the neurological/behavioral
basis of unified consciousness would be very different in different
people. The isomorphism hypothesis would be false, attractive though
it has been to many people.

Dennett’s and the Churchlands’ views fit naturally within
a dynamic systems view of the neural implementation of cognition and
consciousness, the view that unified consciousness is a result of
certain self-organizing activities in the brain and interactions
between brain and world. Dennett thinks that, given the nature of the
brain, which is nothing more than neurons sending and receiving
signals to and from other neurons, consciousness could not take any
form other than something like a pandemonium of competing bits of
content. The Churchlands don’t agree with Dennett about this.
They see consciousness as a state of the brain, the
‘wetware’, not a result of information processing, of
‘software’. Both sides in this debate agree that it is
unlikely that the processes that subserve unified consciousness are
sentence-like or language-like.

9. Conclusions

A great deal of work has been done on the unity of consciousness in
the past few decades. Our introduction to it has been grouped around
the following themes:

In some form, the unity of consciousness is a pervasive,
cognitively important feature of our kind of mind.

Even phenomenal unity of consciousness at a time comes in a number
of forms and consciousness is also unified across time.

The ways in which unified consciousness can break down raise
interesting questions about the phenomenon and throw important light
on its structure.

The topic connects to a number of important issues concerning the
relationship of consciousness to cognition, including whether unified
consciousness across time plays a role in personal identity (the
philosophers’ concept of personal identity).

All the leading theories of the unity of consciousness face
problems.

The state of theorizing on the topic suggests that there is still
much room for further work.

Kant, Immanuel, 1781/87, Critique of Pure Reason, P.
Guyer and A. Wood (trans. and eds.), Cambridge: Cambridge University
Press (cited as Axxx for the first edition of 1781 and Bxxx for the
second of 1787).

Searle, John R., [2000] 2002, “Consciousness”, in his
Consciousness and Language, Cambridge: Cambridge University
Press. Originally published in 2000, Annual Review of
Neuroscience, 23: 557–78.
doi:10.1146/annurev.neuro.23.1.557

Other Internet Resources

Bibliography on the unity of consciousness
on
PhilPapers,
David Chalmers & David Bourget (eds), is the most comprehensive
bibliography on the topic (391 entries, April 2017). The bibliography
for the present entry contains references not included there but that
bibliography refers to many works to which no reference is made
here.